Heat exchanger can't handle solar panel water temperature

"Tempering valve" is used in houses to limit hot water temp at the taps. I don't see why it would not work for this application.

If by heat exchanger you mean a fossil fuel fired pool water heater, most, but not all, heat a relatively small amount of water passing through them, and recombine that heated water with the rest of the inlet water and return an equal volume of what entered the heater to the pool, warmer than it went in.

A solar pool water heater as you will find them will never see an outlet temp. anywhere near 70C. Maybe a few deg. F above the pool water temp., but no more than that.

Solar pool heaters work differently, usually by heating a larger quantity of pool water, sometimes/often the entire pump flow, though flat plate collectors that turn solar energy into heat energy and in so doing transfer that energy to a relatively large amount of water that gets pumped through the collectors with that water getting a relatively small temp. increase, but still returning to the pool at a higher temp. than it left the pool. Fossil fuel fired pool water heaters and solar pool water heaters work differently.

Do not confuse temperature with heat. Adding one degree of temp. to 100 gal. of water will require just as much heat input as adding 100 deg. of temp. to 1 gal. of water. Adding 1 deg. of temp. to 100 gal. of water will be an easier task from an efficiency standpoint.

The fossil fueled heaters function as they do because of fluidmechanical, heat transfer and thermodynamic considerations.

Solar pool heaters do it as they do for reasons having more to do with something called Entropy as manifested by enabling lowered thermal losses to the environment by high flowrates of heated fluid, thus keeping the temp. increase lower, but adding heat more efficiently than if flowrates were lower (and thus water temps. higher) with the accompanying higher thermal losses to the environment.

If you feed the output from a solar pool heater to a fossil fuel fired heater, provided the flowrate into the fired heater is sufficient, you should have no problems, but the two devices operated in series may not be as cost effective as operating them in parallel, or with the fossil fueled heater's turn on temp. higher than the approx. solar pool heater return temp. (and so maybe above the desired pool water temp.). Two heaters make the control logic of maintaining desired water temp. a bit more tricky than either device on its own.

Get a good pool cover and use it. You'll save the most money, both on equipment and fuel. Been there. Done that. The motorized and tightly fitting cover eliminated ~ 2/3 to 3/4 of an inground pool's heat loss in Albuquerque.

FWIW, in effect, a solar pool heater is a heat exchanger of a specialized type and designation, but not really. It sort of defies description a bit in that sense. But one perhaps poor analogy is that it might loosely have some thermal-hydraulic analogy to a mix/mash of some of the elements of what are called transformers and/or gyrators in the areas of modeling of dynamic systems. But that's way off topic.

Thanks for the dtailed reply. However, I obviously did not make my original post clear.

I intended using the solar panel flow as a closed loop, passing over a heat exchanger (the solar flow never mixes with the pool water). The pool flow also goes via the heat exchanger but does not mix with the solar circuit (like an indirectly heated hot water tank).

In this configuration the input and output temperatures of the pool water are not the issue. It is the input temperature of the flow from the solar panels to the heat exchanger.

I think I understand what you have in mind. If the goal is to heat pool water by taking solar heated water (or other fluid) and heating pool water with it via a heat exchanger, then, for a lot of reasons, that's a very inefficient way to accomplish the task. It can sure be done as you have in mind, but you better have some deep pockets. To a first approximation, and partly because of the temp. differences involved and the design you have in mind, the heat exchanger you will need will require about the same or more surface area as the solar collector you have in mind. It will also need to of a design type that can be disassembled (if not replaced) on a regular basis due to the fouling from mineral deposits that will precipitate out of the pool water.

Read up on common methods of heating pool water. Simple technology and quite low tech is commonly that's quite effective for pool water heating applications.For most such applications, unless you are operating the solar pool collector when freezing conditions are likely/common, what you have in mind could be called a misapplication.

Good luck.

It might, but probably not or most likely unnecessary and unworkable for any reasonably designed system for heating swimming pool water. The goal of heating pool water works best, most efficiently, most practically and most economically when the fluid going through the collectors is as close to the pool water temp. as possible (and BTW, sometimes/often below ambient air temp.) Tempering valves will not be needed or even work properly with such small temp. differences.

Op asked how to lower water temp from a solar hot water system so I offered an option. My system often boils and I run a tampering valve at 60c, I think?, and it works fine.

I did not think it would be efficient way to heat pool water but op did not ask for opinions on that.

I

My understanding of what the OP wants or has at this time:

He wants to heat swimming pool water with output from an (existing ?) solar water heater of some as of now unspecified size, type and orientation. Apparently, the solar heater is operating at an elevated temp. at least some of the time that's greater than 70 C. The OP thinks a HX can be used to lower that temp. to 70 C.

Aside from the OP, and apparently you, not understanding why inlet temps., but most often minimum delta T's (temp. differences between two fluids in a HX) are often specified, there might be some value and a better design attained by rethinking what's the best way to meet the OP's needs. On the min. temp. the OP writes of: In short, too small a delta T will mean the HX will have what's probably a lower probability of not meeting the duty (the required amount of heat transfer). Actually, and often but not always, a lower delta T results in a somewhat safer design at the expense of a lower duty (in this case, lower heat transfer with less heat going to the pool water).

Sure, a tempering valve may work. But, depending on the duty (the required range of daily heat input to the pool), and the size of the (existing ?) solar thermal system, it may or may not be useful or practical or even possible to use a tempering valve.

Before any talk of a tempering valve, a lot needs to be known about the OP's situation, the existing equipment and what's available on site to reuse/modify.

For all the hassle and cost including the separate hassle of even preliminary sizing and design of a suitable HX, not to mention it's cost and service/maint./cleaning requirements, it'll likely be more cost effective to consider a separate solar thermal pool water heater.


As for your boiling heater - and I'm telling you this as a retired P.E. who claimed proficiency in boiler and pressure vessel design and also claimed knowledge of solar energy applications as they relate to HVAC and power generation applications : Whether you choose to recognize it or not, have a potentially dangerous situation on your hands.

Get it checked out.

Before we all get in a twit, perhaps it might be useful to find out a little more from the OP before we drill so deep into Carnot's theories. It seems someone here is trying to define the problem to fit his skillset and the result is another thread hijack.

And why do you think I stated what I think I understand of the problem from what the OP has shared so far if not to get a better understanding from the OP of what he has in mind ?

As I stated previously, except to point out errors as I see them in your statements, I'm not going to waste time on commenting to your stuff, beyond stating I probably forgot more about the matters of this thread than you will even know or even imagine.

In the spirit of pointing out your errors as I see them and that it also seems a bit of maybe the pot (you) calling the kettle (me) black on a thread hijack, I'd call attention to your incorrect and off thread reference to Carnot.

I'd point out that while his better known and essentially only work is indeed one of the foundations of classical Thermodynamics, Carnot's work dealt with the maximum theoretical efficiency of heat engines running on a theoretical cycle and, believe me on this, has nothing to do with the matters of this thread. For the most part the real meat of matters of this thread deal with heat transfer, not Thermodynamics. That's a difference I'd suggest is unknown to you and maybe part of the cause of your ignorant and irrelevant reference to Carnot who, BTW, had only one theory (not plural as you mistakenly state) on the efficiency of heat engines and little else, especially of the finer points of heat transfer and it's mechanisms which is what most of thes thread is about, particularly as it relates to heat exchangers. Others later refined and formalized Carnot's seminal work but that's off topic.

You may not care, and I sure don't, but you just embarrassed yourself to anyone who does know anything about the subject. You know squat about Carnot and why Thermodynamics has nothing to do with this thread. If there's a hijack, it's on you. Get in a twit if you want, but screw up by throwing crap around that's out of your league and expect to get called on it.

I'll be more than willing to answer any questions the OP may have about his system, how it operates, or some informed opinions about how to perhaps get to the stated goals as safely, efficiently and economically as possible.

You are correct I do not have in depth knowledge of what is the best way to heat a pool, that is why I did not offer any advise other than what I did. I have followed many of your posts have no doubt your know your stuff and would reply to the thread.

For my hot water systems I have had two different types the Solarhart forst then the Edwards now, and both will bring water to boiling. They have both been 300l systems and with only two of us in the house and living in the tropics there is ample sun to heat water.

The first one was a Solahart 302SP

The 302SP Series is a 300lt roof-mounted, closed-circuit system utilising the new revolutionary Envelope solar collectors.


The one I have now is an Edwards brand, the Edwards brand is now owned by Rheem. This one is basically the same as the Solarhart one but it has a stainless tank and is an open loop system. We have good water and dont get frost so no need for a closed loop system.

They both have safety valves standard that let pressure out if needed. Both are commercial systems and have been fitted as per instructions so I assume they are safe??.

Before we all get too heated (sorry about the pun), here is what I now propose having read all the responses and looked at other options.

I have 2 Bosch Worcester (as they are branded in the UK) solar panels FKC-1S, and all the associated gear so wish to make the best use of it. My current thinking is this.

Using a dual-coil indirect tank (one purpose designed as a heat store), probably 500 litres, I can heat the water in the tank using standard closed loop circuit from the panels, via the tank and back to the panels. I'll put a heat dump on the closed loop. This means the tank water heats up when the panels get the most direct light and the heat is stored for the night time when the pool pump runs, the electricity is cheaper, and the pool heat loss is greatest.

Using the second coil in the tank I will set up a closed loop from that coil to the pool heat exchanger. As its a pool heat exchanger, its designed to deal with chlorinated water. As before, the pool water never mixes with anything other than pool water and taking J P M's advice about efficiency, I can keep the heating water to the heat exchanger, at an optimal temperature.

Any excess in the heat store, I will use to feed my electrically heated domestic hot water tank.

Will this work, am I crazy, or can you suggest better use of these panels (which I already own, long since paid for) which I hope to benefit from?

If I had two of those panels and a pool I would do the same. If you have plumbing skills, and can program the controllers, you can accomplish the goals that you set out. Or you can hire those services. The concept is to store energy from the sun and use it to top off your pool and use any excess energy to preheat your domestic hot water.

In my way of thinking, anytime you can use free energy from the sun you are getting a benefit. You are saving money on whatever you would otherwise spend on purchased energy. You have probably researched the costs of doing that and you undoubtedly know the cost of energy that you would otherwise purchase. As others may point out the devil is in the details but you get to decide if the payback is acceptable to you. There are a number of concepts of efficiency that can be used to justify or negate your concept.

Questions: How much water is in the pool ? And, is it above/below ground ?

Here's the deal: Yes it will probably work if done safely and in a well designed fashion. Whether or not the results will be noticeable either in terms of how much the pool temp. goes up or how much the pool heating bill go down as a result of what sounds like a lot of work and added maint. is a different question.

Trying to be professional about this, if you have ~ 4 m^2 of solar domestic water heater surface as I think 2 ea. of the FKC-1S panels have (at least as best as I can surmise with the limited info I could find), you can do what you want, but for all the heat you'll get out of them, it may not be noticeable as an increase in the pool water temp., or as much of a reduction in the fuel used to keep that water at desired temp.

My guess is that you are unaware of the limitations of solar energy to provide the quantity of heat necessary to maintain a desired pool water temp. or effect a noticeable change in pool water temp. or the means available to achieve any form of safe and cost effective solution that may be currently available. Read up on solar pool water heating requirements and what's available for a primer on the scope of what you want to do.

I'll get you started: As a rough rule of thumb and a dart throw for preliminary design, a solar pool water heater usually needs to have a surface area of very roughly and approx. 2/3 or so of the surface area of the pool. That's when used in relatively warm summer conditions. And that's for pool water heaters made for the purpose. Such heaters operate at very low temps relative to what solar flat plate collectors operate at, and because of that low temp., operate without glazing and are usually constructed frameless. As an aside, and because of those factors including the low operating temps, such collectors operate at relatively high efficiencies of something approaching. Because of all that, such collectors are less expensive than panels used for potable water heating as the Bosch equipment you have.

Bottom line: Depending on the size of the pool - actually the daily or design heat load in terms of the daily heat loss from the pool that needs to be replaced - your solar collectors are probably: 1.) Quite undersized for the task. 2.) Not, or at best less appropriate for what you want to do. 3.) Fixed in size and so fixed in long term annual output, meaning any heat you do add to the pool that gets supplied by those collectors will come at the expense of a lower quantity of heat that gets supplied to the potable water heating system which I'm assuming here is the intended and original duty of the Bosch system.

That is, on an annual basis, you will have, for all intents and purposes a fixed amount of heat from the solar thermal system you have. So, the panels will only supply so much heat. If the current solar water heating system is oversized, maybe at some times of the year, you might make a case for shunting the excess solar heat to the pool during those times of excess generation, but for the added expense, I'd sure be doing a lot of homework to try to ensure I wasn't spending a bunch of time, toil and treasure on a scheme that will produce little more than maint. problems and maybe a few pounds sterling/yr.

Put bluntly but straight, and not meant as anything more than opinion and information, your ignorance of the situation is causing you to think some things are viable and practical that most other folks more experienced and knowledgeable in such matters know to be the folly of such ignorance. That ignorance is the enemy here. Get educated. Based on the limited information you supply, my guess is what you suggest above, or some form of it, will work, but my opinion is that it will not come to a good end or produce measurable results or anything near what may be the desired outcome. But don't shoot me, I'm only calling balls and strikes as I see'em.

Take what you want of the above. Scrap the rest.

Good Luck

Besides your units being batch heaters, which is fine, I don't know what your particulars are, but if I was you, and as a cheap and expeditious safety measure, I'd first (and soon) cover a portion of the inlet side (the bottom) of one or both the collector surfaces during times of the year when high daily insolation is likely to the extent that the system temp. and pressures stay below the set points on the relief valve.

Using a P & T. relief valve as a regulating device is not a good idea. If a high solar input causes the relief valve to see either a pressure and/or temp. that's too high, the valve will lift (operate/open). That's what it's supposed to do. But that action is meant to cover upset or unusual conditions (such as stagnation from blocked or no flow conditions for example), not as a daily or regular temp. regulating mechanism or control method. Doing so is sort of analogous to using the emergency brake on a vehicle as an everyday means of slowing the vehicle down. Besides, and is likely with common P. & T. valves, too much use (and it doesn't take much) will cause the valve to leak and worse, maybe get stuck, unable to either open or close fully. If/When that happens, and it will, you'll have a safety issue on your hands.

The systems may be plumbed and set up per instructions, but those instructions do not cover all operating conditions. Cover a portion of the collector surface with something like a corrugated fiberglass roof panels found at big box and affix them with bungee cords.

Back on topic to the OP's thread.